1. Field of the Invention
The present invention relates to antenna devices and communication terminal apparatuses. In particular, the present invention relates to an antenna device preferably for use in an RFID tag or a reader/writer that operates in an HF band, and a communication terminal apparatus including the antenna device.
2. Description of the Related Art
RFID (Radio Frequency Identification) systems are widely used as billing systems or article management systems. In an RFID system, a reader/writer and an RFID tag are caused to perform wireless communication in a noncontact manner, and radio-frequency signals are transmitted and received between these devices. The reader/writer and the RFID tag each include an RFID IC chip for processing radio-frequency signals and an antenna for transmitting and receiving radio-frequency signals. A coil antenna is used as an antenna in the case of, for example, an HF-band RFID system utilizing a 13.56 MHz band. The coil antenna on the reader/writer side and the coil antenna on the tag side are coupled to each other via an induction magnetic field.
In recent years, HF-band RFID systems have been utilized for communication terminal apparatuses such as mobile phones, and the communication terminal apparatuses have been used as readers/writers or RFID tags in some cases. In these cases, an RFID IC chip is mounted on a printed wiring board, and an antenna is adhered to a terminal housing or is provided in a free space in the terminal housing. Thus, the RFID IC chip and the antenna are connected to each other in a direct-current manner via a flexible cable or a contact pin.
On the other hand, as disclosed in, for example, Japanese Patent No. 4325621, a configuration is known in which a feed coil connected to an RFID IC chip is mounted on a control board, and the feed coil is magnetically coupled to a coil antenna provided on an antenna board. With this configuration, a radio-frequency signal can be transmitted from the feed coil to the coil antenna via a magnetic field, and thus the RFID IC chip and the coil antenna can be connected to each other without using a flexible cable or a contact pin.
As in the antenna device disclosed in Japanese Patent No. 4325621, in a case where a feed coil and a coil antenna are magnetically coupled to each other and signals are transmitted accordingly, a mutual inductance is generated between the feed coil and the coil antenna, and thus the impedances or resonance frequencies may be different from each other inconveniently, depending on the positional relationship therebetween. In particular, if the feed coil and the coil antenna are directly and magnetically coupled to each other and if the degree of coupling is extremely high, the resonance points of the feed coil and the coil antenna are separated from each other even if the resonance frequencies of the feed coil and the coil antenna correspond to a carrier frequency. Thus, the signal transmission efficiency from the feed coil to the coil antenna, or the signal transmission efficiency from the coil antenna to the feed coil decreases, and as a result, the communication distance decreases.